The size of an array can be calculated using the sizeof operator. It returns the total size of the array in bytes, including the storage occupied by all its elements.

#include <stdio.h>

int main(void)
{
    int numbers[] = {10, 20, 30, 40, 50};

    size_t total_size = sizeof(numbers);
    size_t element_size = sizeof(numbers[0]);
    size_t element_count = total_size / element_size;

    printf("Array size: %zu bytes\n", total_size);
    printf("Element size: %zu bytes\n", element_size);
    printf("Number of elements: %zu\n", element_count);

    return 0;
}

Example Output #

On an implementation where sizeof(int) is 4 bytes:

Array size: 20 bytes
Element size: 4 bytes
Number of elements: 5

Explanation #

The expression:

sizeof(numbers)

returns the total size of the array in bytes.

The expression:

sizeof(numbers[0])

returns the size of one array element.

Dividing the total array size by the size of one element calculates the number of elements:

sizeof(numbers) / sizeof(numbers[0])

This calculation works when numbers is an actual array object, rather than a pointer to an element.

Array Size Expressions #

The sizeof operator returns a value of type size_t, which should be printed using %zu.

Important: When an array is passed to a function as a parameter, it is adjusted to a pointer parameter. Inside that function, sizeof returns the size of the pointer, not the size of the original array. The array length must be passed separately when needed.